1. Field of the Invention
The present invention relates to a method of estimating the internal state of an electrochemical system for electrical power storage, such as a battery (lead, Ni-MH, Li-ion, etc.). The method allows management of batteries used in stationary or on-board applications, notably during the operation thereof.
2. Description of the Prior Art
The battery is one of the most critical components in the case of hybrid or electrical vehicle applications. Proper operation of these applications is based on a smart battery management system (BMS) whose purpose is to operate the battery with the best compromise between the various dynamic demand levels. The BMS requires precise and reliable knowledge of the state of charge (SoC) and of the state of health (SoH).
The SoC of a battery corresponds to its available capacity and it is expressed in percentage of its nominal capacity given by the manufacturer, or in percentage of its total capacity measured under given conditions when such measurement is possible. Knowing the SoC allows estimation of the time during which the battery can continue to supply power at a given current before the next recharge, or until when it can absorb recharge before the next discharge. This information conditions the operation of the systems using batteries.
During the life of a battery, its performances tend to degrade gradually because of the physical and chemical variations that occur while it is being used, until it becomes unusable. The SoH represents the state of wear of a battery. This parameter corresponds to the total capacity of a battery at a time t during its life, and it is expressed in percentage of the total capacity determined at the life start, which is equivalent to the nominal capacity given by the manufacturer, or to the capacity measured at the life start under given conditions.
Precise and reliable estimation of the SoC and of the SoH, for a vehicle, allows for example prevention of the vehicle supervisor from behaving too cautiously regarding the use of the battery energy potential, or conversely. A wrong diagnosis of the state of charge can lead to an overestimation of the number of kilometers that can be traveled and put the motorist in a difficult position. Good estimation of these indicators allows avoiding battery safety oversizing, thus allowing saving weight on board, and consequently consumption of fuel. SoC and SoH estimation also allows reduction of the total cost of the vehicle. A correct SoC and SoH diagnosis tool thus guarantees efficient, reliable and perennial battery capacity management over the entire operating range of the vehicle.
Several methods for estimating the SoC and the SoH of a battery are known.
There are for example the known coulomb-counting or book-keeping methods. These methods however lead to estimation of errors by disregarding phenomena such as self-discharge. There is also a known method wherein the no-load voltage is measured as the SoC indicator. Using other indicators, such as the estimation of an internal resistance for example U.S. Pat. No. 6,191,590 B1 and EP Patent 1,835,297 A1, is also a known method.
These two methods are characterized by the fact that the SoC is first associated with one or more measurable or easily estimable quantities (potential, internal resistance), through static maps or analytical functional dependencies. However, these dependencies are in reality much more complicated than what is normally taken into account in the BMS, which often leads to SoC estimation errors.
Another approach is based on mathematical battery models using estimation techniques known in other spheres. U.S. published patent application 2007/0,035,307 A1 notably describes a method of estimating the variables of state and the parameters of a battery from service data (voltage U, current I, T), using a mathematical battery model. The mathematical model comprises a plurality of mathematical sub-models and allows faster response. The sub-models are models of equivalent electrical circuit type, referred to as RC models, associated with restricted frequency ranges.
A potentially more promising method is based on the measurement, by impedance spectroscopy (EIS), of a quantity parametrized by the SoC. For example, U.S. published patent application 2007/0,090,843 determines by EIS the frequency f± associated with the capacitive/inductive transition. A correlation between frequency f± and the SoC is presented for a lead battery, as well as for Ni—Cd and Ni-MH batteries. A similar approach is based on the modelling of the EIS spectra by equivalent electrical circuits, whose components are parametrized by the SoC, as described in U.S. Pat. No. 6,778,913 B2, which allow development of an automotive battery tester Spectro CA-12 (Cadex Electronics Inc., Canada) based on the multi-frequency electrochemical impedance spectroscopy for the acid-lead pair. The EIS spectra are approximated by equivalent electrical circuits and the evolution of the components is parametrized by the SoC. Similarly, in U.S. Pat. No. 6,037,777, the state of charge and other battery properties are determined by measuring the real and imaginary parts of the complex impedance/admittance for lead batteries or other systems. The use of RC models is also described in EP 880,710, the description of the electrochemical and physical phenomena at the electrodes and in the electrolyte serving as a support for the development of the RC model, the temperature of the battery being simulated by the model, in order to increase in precision, in relation to an external measurement.
Concerning the SoH estimation methods known in the literature, in WO 2009/036,444, the authors introduce a reference electrode in commercial elements in order to observe the degradation reactions of the electrodes. This method however requires much instrumentation, notably for inserting a reference electrode in the element, and more complex electronic management of the battery.
French Patent 2,874,701 describes a method using a temporal electric perturbation in order to compare the response obtained with a reference response. However, this method is more difficult to implement for Li-ion type elements whose response variations after this type of perturbation are very low and can therefore not allow precise SoH measurement.
Impedance analyses have also been described in the literature. U. Tröltzsch et al. (Electrochimica Acta 51, 2006, 1664-1672) describe a method wherein they use impedance spectroscopy coupled with the adjustment of impedances according to an electrical model so as to obtain the state of health of the element. This technique however requires stopping using the element for the measurement.
The invention thus is an alternative method of estimating an internal state of an electrochemical system for electrical power storage, such as a battery, without modelling by an equivalent electrical circuit. The method is directly based on an electrochemical impedance model allowing defining this impedance as a function of the SoC for example, and parameters. These parameters are calibrated by an adjustment on electrochemical impedance measurements obtained beforehand for different internal states.